... Solar System . ... Sun . ... Comets . ... Some get thrown out of the solar system forever, whereas others end up on yet smaller orbits, visiting the inner solar system fairly frequently - such as comet Halley. ... When a comet moves into the inner solar system, this is heated by the Sun, and the ices turn into gas which forms the head or coma of the comet, which may be hundreds of thousands of kilometres across and visible from Earth. The most striking part of many active comets is the tail . ...